The present invention relates to a system for controlling the clutch torque of an electromagnetic clutch disposed between an engine and a transmission of a motor vehicle, and more particularly to a system for controlling the variation of the clutch torque from a high clutch torque while an accelerator pedal is depressed to a low clutch torque at releasing the pedal.
Japanese Patent Application Laid-open No. 57-30624 discloses a clutch control system, in which clutch torque is maintained to a value larger than engine torque by a rated current as long as the vehicle is driven faster than a predetermined speed, irrespective of the condition of the accelerator pedal. In this construction, when the accelerator pedal is released, irregular and abrupt fluctuation occurs in engine torque which is transmitted to a driving system, giving an unpleasantly shocking the driver. Further, because a large amount of rated current is needed to be supplied to the clutch under the condition of lower engine speed while the acceleration pedal is being released, the electric power of a battery is greatly consumed.
In order to remove such drawbacks, as shown in FIG. 4, it was tried to absorb the torque variation by lowering the clutch torque to such a low value T.sub.2 as long as the engine braking could be effected while the accelerator pedal was being released, although the clutch torque during the depression of accelerator pedal was fixed to a high value T.sub.1, as was in the conventional art. However, by such stepwise variation in clutch torque, the transmission of the fluctuation of engine torque can not be sufficiently prevented.
On the other hand, a dashpot device has been conventionally pr9ovided for retarding the closing speed of a throttle valve when releasing the accelerator pedal in order to absorb the shock caused by the engine torque change. However, it takes a long time to lower the engine speed to an idling speed after the acceleration pedal is released, which results in high fuel consumption, and in the reduction of engine braking force, as well as complicating the construction because of the provision of a dashpot.